Field of the Invention
The present disclosure relates to techniques for acquiring acoustic data. More particularly, the present disclosure relates to obtaining improved seismic signal data by controlling an activation time of the seismic source.
Background of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
Measurement-while-drilling involves the sensing of one or more downhole parameters during the drilling process. Sensors, typically mounted within drill collars located above the drill bit, are used to obtain information regarding the drilling process or subsurface conditions. A subset of these measurements may be transmitted to the surface, often using an acoustic or “mud pulse” telemetry system. Other measurements may be stored in recording devices located within the drill collars. This data can be retrieved when the drill bit is raised (also called “tripped”) to the surface.
Seismic measurement-while-drilling data is acquired using seismic sensors, such as geophones or hydrophones that are typically located within a drill collar positioned above the drill bit. Since the drill bit generates a tremendous amount of noise, it is typical to collect seismic data, which may be generated by an uphole source, only during the time the drill bit or drill string is not moving or drilling. This usually equates to the time during which the drill string or drilling is temporarily stopped in order to add or remove pieces of drill pipe to the drill string at the surface.
As mentioned above, communication between the tool and the surface is typically accomplished with a “mud pulse” or other low data telemetry system, making it impractical for the operator to send commands to the tool during drill stoppage to regulate the time sequence or activity related to the seismic data acquisition. Furthermore, due to the vast amount of data that is acquired from the seismic signal, it is also impractical to collect seismic signal data for large periods of time, such as from the time the drilling ceases to the time it commences. To overcome these limitations, the seismic recording system in the tool is programmed to start and stop recording for specific time periods and at specific intervals. In conjunction, the source is activated relative to the programmed time periods to ensure that the seismic signal arrives at the tool during the recording period.
It has been found that there are preferred locations within the recording period where the signal should arrive. This has mostly to do with the processing of the seismic signal once received. It has also been found that as the tool traverses through the earth formation (as the tool is drilling for example), the distance between the tool and the source, and the velocity profile of the formation there between, changes causing the seismic signals to arrive at the tool at undesired times within the period and, more drastically, to miss the period altogether.